Controllable semiconductor structure with improved switching properties

ABSTRACT

The invention concerns a controllable semiconductor structure comprising a base region ( 101, 201, 301, 401 ), a source region ( 106, 212, 312, 412 ) and a drain region ( 107, 213, 313, 413 ) a conductive duct being provided in the base region between the source and drain. According to the invention, the duct can be constricted by regions lying parallel thereto, an active control region ( 102, 202, 302, 402 ) and an opposite passive control region ( 103, 203, 303, 403 ) which each form a blockable passage with the base region ( 101, 201, 301, 401 ). Further provided is a conductive connection ( 108, 209, 309, 409 ) between the passive control region ( 103, 203, 303, 403 ) and the source region ( 106, 212, 312, 412 ), the semiconductor material of the base region ( 101, 201, 301, 401 ) having an energy gap of more than 1.2 eV.

This is a 371 of PCT/EP97/05080, filed Sep. 17, 1997.

BACKGROUND OF THE INVENTION

The invention relates to a controllable semiconductor structure havingimproved switching properties.

The literature describes numerous component structures referred to asJFET or MESFET, in which the conduction properties are controlled by thevoltage-dependent expansion of one or more space-charge zones (pntransition in JFET, Schottky transition in MESFET). The base structurewas first proposed by W. Schockley: A Unipolar ‘Field-Effect’Transistor, in the Proceedings of the I.R.E., 1952. Instandard-technology conversions, as are described in W. von Münch,Einfuhrung in die Halbleitertechnologie [Introduction to SemiconductorTechnology], Teubner, 1993, large parasitic capacitances (especiallyinput capacitance and reverse-transfer or Miller capacitance) occur,leading to low limit frequencies in amplifiers and causing longswitching times, and therefore large switching losses, in switchingapplications. This is also the case for high-blocking JFETs that operateaccording to the RESURF principle, for example, as described in U.S.Pat. No. 4,422,089; in these JFETs, the field-intensity peaks at thecomponent surface are reduced by a suitable selection of the doping anddepth of the lateral drift zone.

It is known from textbooks, e.g., R. Paul: ElektronischeHalbleiterbauelemente [Electronic Semiconductor Components] that JFETsand MESFETs are usually produced on, for example, insulating,semiinsulating or insulated substrates (e.g., the SOI technique orsapphire in silicon, highly-compensated material in gallium arsenide,etc.) to minimize the parasitic capacitances.

These techniques have the following disadvantages:

1) Because of the insulating, semi-insulating or insulated substrate, nocurrent flow can occur in the vertical direction. Therefore, no verticalcomponents can be produced with this method, which limits its use forpower components.

2) The production of wafers with an insulating or insulated substrate iscomplicated and expensive. In addition, problems due to, for example,temperature limitations can occur in the further processing.

3) In semiconductors that cannot be rendered semi-insulating throughcompensation, a second material must be used as an insulator. This leadsto, on the one hand, stress because of different thermal expansioncoefficients and, on the other hand, more intense internal heating ofthe components because of the generally lower thermal conductivity ofthe insulator. Furthermore, the crystal quality of the activesemiconductor layer is frequently worse in heteroepitaxial production onan insulator than in homoepitaxially-produced layers because oferroneous lattice adaptation.

4) The insulation technique can only be combined with the RESURFtechnique in thin insulator layers, which in turn increases theparasitic capacitances.

It is therefore the object of the invention to use simple technologicalmeasures and few steps to create a semiconductor structure that has agood blocking effect, and permits higher limit frequencies and lowerswitching losses than conventional components.

SUMMARY OF THE INVENTION

The above object generally is achieved according to the presentinvention by a controllable semiconductor structure having a baseregion, a source region and a drain region, with a conductive channelbeing provided in the base region between the source and the drain, andwherein the channel can be pinched off by zones parallel thereto,including an active control zone and an oppositely-located passivecontrol zone, which respectively form a blockable transition to the baseregion; a conductive connection is provided between the passive controlzone and the source region; and the semiconductor material of the baseregion has a band gap of more than 1.2 eV.

The drawing illustrates embodiments of the invention.

A BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic showing of the base structure

FIG. 2 is a schematic showing of a first embodiment (implantation)according to the invention

FIG. 3 is a schematic showing of a second embodiment (epitaxy) accordingto the invention

FIG. 4 is a schematic showing of a vertical component, as a thirdembodiment according to the invention and

FIG. 5 is a graph showing the result of a simulation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a schematic, sectional representation of the structure of theinvention, which comprises a semiconductor region 101 of a firstconductivity type as a base material, which is bordered at two locationsby non-touching regions 102 and 103, which are referred to as active andpassive control zones, and respectively form a blockable transition withthe semiconductor region 101, and are electrically contacted by theelectrodes 104 and 105. Furthermore, the two edges of region 101 thatare not bordered by the two control zones 102 and 103 are electricallycontacted, at least in a region, by the electrodes 106 and 107. In thethird dimension, the structure of the invention has noelectrically-conductive path between the electrodes 106 and 107 thatcannot be influenced by the zones 102 and 103.

The structure of the invention is characterized in that the contacts 105and 106 are electrically connected by a layer 108, while the contacts104 and 105 can have different potentials, unlike in conventionalstructures, and the semiconductor material 101 has a band gap largerthan 1.2 eV (at room temperature).

Examples of materials considered as the semiconductor material aregallium arsenide, the different polytypes of silicon carbide, galliumnitride, diamond and aluminum nitride.

Independently of one another, the regions 102 and 103 can comprisedifferent semiconductor materials of the same semiconductor material asthe region 101, or comprise a metal. If the zone 102 or 103 comprises asemiconductor material, it must possess the opposite conductivity typeof the region 101. If the zones comprise metal, the metal must form aSchottky transition with the base material of the region 101.

The expansion of the space-charge zone around the zone 102, and thus thecross section of the conductive channel in the base material between theelectrodes 106 and 107, can be controlled by the application of avoltage between the electrodes 104 and 106. If the voltage between theelectrodes 104 and 106 becomes so large that the space-charge zones ofthe opposite regions 102 and 103 touch, the conductive channel between106 and 107 is interrupted and the connection between them becomeshighly-resistive. Generally, in this operating state, a current flowthat increases strongly superproportionally with a further increasingvoltage between the electrodes 104 and 106 occurs between the electrodes104 and 105, and thus via the conductive connection 108 to the electrode106, and possibly leads to the destruction of the component or theoverload of the control generator.

The invention, in contrast, is based on the realization that thedifference between the control voltage during pinch-off of theconductive channel and the control voltage when this current is set canbe influenced by the band gap of the semiconductor material in theregion 101.

The precise value of this control-voltage difference is essentiallydetermined by the energy gap of the semiconductor material with a givenstructure and doping. It is, however, also dependent on othersemiconductor properties, especially the relative permittivity.Therefore, no clear connection can be established between thecontrol-voltage difference and the band gap. As has been discoveredthrough simulations, however, a semiconductor material having a largerband gap also tends to lead to a larger difference between the controlvoltage when the conductive channel is pinched off and the controlvoltage when the current is set via the control connector. Theconnection 108 can therefore only be used in association with acorrespondingly-selected semiconductor material (wide band-gapmaterial), and would lead to a high control-power requirement, or evencritical operating states, in silicon, for example.

In the structures known up to now, this large control-power requirementcould only be avoided with a short-circuit between the electrodes 104and 105, which rules out the connection 108, and the parasiticcapacitances become very large. In the structure of the invention, incontrast, the parasitic capacitances between the electrodes 104 and 106(minimum input capacitance) or between 104 and 107 (reverse-transfercapacitance) can be minimized essentially by a small expansion of theregion 102 of the active control zone. The capacitance between theelectrodes 105 and 106 is practically short-circuited by the connection108, and is thus virtually ineffective. The capacitance between theelectrode 105 of the passive control zone 103 and the drain electrode107 is insignificant for most applications, because it is recharged bythe connected load circuit, and not by the control circuit, as inconventional structures.

In addition to the above-described structure, in which a conductivechannel is disposed between the electrodes 106 (source) and 107 (drain)without the application of a control voltage, that is, current can flow(“normally-on”), a “normally-off” structure can also be produced in asuitable design. For this purpose, the spacing of the active controlzone 102 from the passive control zone 103 must be so small, or thedoping of the base region 101 must be selected to be so low, that thestatic space-charge zones around the control zones 102 and 103 (i.e.,without the application of a control voltage between the electrodes 104and 106) already touch.

Overall, the structure of the invention constitutes an intermediatesolution between JFET and MESFET, in which no control electrode can beshort-circuited with the load-circuit electrodes, and the so-calledcurrent limiter, in which all control electrodes are short-circuitedwith a load-circuit electrode.

A variation of the structure of the invention that deviates further fromthe so-called FCTh (Field-Controlled Thyristor) or SITh (StaticInduction Thyristor) includes a semiconductor zone 100, which extends infront of the drain electrode 107 and has the opposite conductivity typeof the base region 101. With a current flow between the electrodes ofthe source and drain (107 and 106) through this zone, minority carriersare injected into the base region 101, thereby increasing theconductivity there. Because at least the threshold voltage of thetransition between the region 101 and the additional zone 100 must beovercome, this structure is particularly well-suited for high-blockingcomponents. Moreover, a further zone 116 can be disposed between thisadditional zone and the region 101, the additional zone having the sameconductivity type as the region 101, but a heavier doping. This zoneimproves the blocking capability of the component.

The following advantages ensue from the invention:

1) The structure can be used to produce vertical components (see thethird embodiment).

2) No special wafers or technological steps are required as the startingmaterial for producing this structure (see embodiments).

3) No high thermal resistances are present due to additional insulatorlayers. Therefore, only a comparatively low internal heating occurs.

4) It is completely compatible with the RESURF technique, and thereforealso suitable for high-blocking components.

5) The parasitic capacitances can be made very small, or made to be onlyof secondary importance for operation, or even be completely omitted, bythe layout or technical measures. This attains higher limit frequenciesand lower switching losses.

EXAMPLE 1

FIG. 2 shows the structure of a lateral component that was producedthrough ion implantation. The starting material is heavily-doped, n- orp-conducting SiC (214). A 10 μm-thick, p-conducting SiC epitaxy layer isapplied to the starting material as a passive control zone 203 with adoping concentration of 10¹⁶ cm⁻³. An n-conducting channel zone or baseregion 201 having a doping concentration of 10¹⁷ cm⁻³ is produced inthis layer through nitrogen or phosphorous ion implantation. In thisregion 201, the heavily-doped, n-conducting source zone and the drainzone 212 and 213, respectively, are produced through nitrogen orphosphorous ion implantation for improving the electrical contacting ofthe base region 201. Finally, the heavily-doped, p-conducting zones, theactive control zone 202 and the contacting zone 211, are producedthrough aluminum or boron ion implantation. The difference in thepenetration depth of the ion implantations of 201 and 202 is about 0.4μm. Then, the implantations are annealed or activated through atemperature treatment preferably between 1000 and 2000° C. Asilicon-dioxide layer 210 is applied for passivating the upper surfaceat which the active control zone 202 and the source and drain zones orregions 212 and 213 are disposed. The active control zone 202 and thecontacting zone 211, as well as the source and drain 212 and 213, aremade accessible through masked etching of this oxide layer, thenmetallized, with the zones 211 and 212 preferably being short-circuitedby a metallization 209.

If a potential that is positive with respect to the electrode 209 isapplied to the drain electrode 207 in the component produced in thismanner, a current flows, without a potential difference between theelectrodes 204 and 209, from 207 after the electrode 209(“normally-on”). Through the application of a potential that is negativewith respect to the electrode 209 to the electrode 204 of the activecontrol zone 202, the space-charge zone in the region around 202 can beenlarged, and the current flow between the electrodes 207 and 209 cantherefore be reduced. Through the application of a potential that ispositive with respect to the electrode 209 to the electrode 204, thespace-charge zone around the region of the active control zone 202 isreduced, and the current between the electrodes 207 and 209 increases.If the blocking layer between the active control zone 202 and the base201 is a pn transition, when the threshold voltage is exceeded, theconductivity in the channel region of the base 201 can be furtherimproved (conductivity modulation) through the injection of minoritycarriers.

EXAMPLE 2

FIG. 3 shows the structure of a lateral component that was produced in asecond epitaxy step. The starting material is heavily-doped, n- orp-conducting SiC (region 314). A 10 μm-thick, p-conducting epitaxy layer303 is applied to the starting material as a passive control zone, witha doping concentration of 10¹⁶ cm⁻³, and a 1 μm-thick, n-conductingepitaxy layer 301 is applied as the base region for the channel zone,with a doping concentration of 10¹⁷ cm⁻³. To contact the layer 303, theheavily-doped, p-conducting zone 311 is produced through aluminum orboron ion implantation, the layer extending from the surface through thebase region 301. In the base region 301, the heavily-doped, n-conductingsource and drain zones 312 and 313 are produced through nitrogen orphosphorous ion implantation for improving the electrical contact of thesource and drain to the base region 301 and its channel zone. Finally,the 0.6 μm-thick, heavily-doped, p-conducting active control zone 302 isproduced through aluminum or boron ion implantation. Then, theimplantations are annealed or activated through a temperature treatmentpreferably between 1000 and 2000° C. A silicon-dioxide layer 310 isapplied for passivating the surface. The control zone 302 and thecontacting zones 311, 312 and 313 are made accessible through maskedetching of this oxide layer, then metallized, with the zones 311 and 312being short-circuited by the electrode 309. The function of thiscomponent is analogous to that of the first embodiment.

FIG. 5 shows the current densities that have been determined with theuse of simulations, and occur at a component having the structure,doping, etc., described in this embodiment, but different semiconductormaterials (germanium, silicon, 6H silicon carbide). The currentdensities are a function of the control voltage, that is, the voltagebetween the electrodes 304 and 309. The load current, in this examplethe current that flows from 313 to 312 with a fixed output voltage of 10V (between the electrodes 307 and 309), is shown as a solid line. Incontrast, the control current, namely the current that flows undesirablythrough the control zone 302 to 303, is shown as a dashed line.

The problems associated with a semiconductor material having a smallband gap can be seen clearly in the example of the germanium component(band gap of germanium at room temperature: E_(G)(300K)=0.66 eV). If,for example, a current density of 10² Acm⁻² is defined as harmless forthe control generator and sufficiently-low for the “off” state of theload circuit, the germanium component cannot be used. With a currentdensity of about 0.1 Acm⁻², the control current is exactly as large asthe load current, and increases approximately exponentially with thecontrol voltage. The silicon component (silicon: E_(G)(300K)=1.12 eV),in contrast, has a voltage difference of merely one Volt, which is,however, by no means sufficient for reliable operation. Only with theuse of silicon carbide (in this case, the 6H polytype with E_(G)(300K)=3eV) is the voltage difference increased to over 4.5 V, which can assurereliable operation.

EXAMPLE 3

FIG. 4 shows the structure of a vertical component of the invention thatwas produced in a second epitaxy step. The starting material is aheavily-doped, n-conducting SiC layer 414. A 10 μm-thick, n-conductingepitaxy layer 415 having a doping concentration of 10¹⁶ cm⁻³ is appliedto this substrate layer 415 414. In this epitaxy layer, the 0.6μm-thick, heavily-doped, p-conducting passive control or shielding zone403 is produced through aluminum or boron ion implantation, thenannealed or activated through a temperature treatment preferably between1000 and 2000° C. A second, 1 μm-thick, n-conducting epitaxy layer 401having a doping concentration of 10¹⁷ cm⁻³ is applied as the baseregion. The heavily-doped, p-conducting contacting zone 411, whichextends from the surface through the base region 401, is producedthrough aluminum or boron ion implantation for contacting the zone 403.In a further embodiment for producing a contacting zone, a window regionis etched off, so the zone 403 is directly accessible from the surface.In the region 401, the heavily-doped, n-conducting zone 412 is producedthrough nitrogen or phosphorous ion implantation for improving theelectrical contact to the base region 401. Finally, the 0.6 μm-thick,heavily-doped, p-conducting active control zone 402 is produced throughaluminum or boron ion implantation. Then, the implantations are annealedor activated through a temperature treatment preferably between 1000 and2000° C. A silicon-dioxide layer 410 is applied for passivating thesurface. The active control zone 402 and the contacting zones 411 and412 are made accessible through masked etching of this oxide layer, thenmetallized, with the zones 411 and 412 preferably being short-circuitedby 409. Moreover, the electrode 407 is produced through themetallization of the rear side.

The notable feature of this structure is the decoupling of the controlregion and the drift region, with the separation being optimized. Thefunction of the control region (channel zone 401, active control zone402 and passive control or shielding zone 403) is analogous to thestructure of the first embodiment. In addition, however, the drift zone415 is present, which must absorb the blocking voltage between theshielding zone 403 and the substrate 414 during operation. With smallblocking voltages, the upper control region of the drift zone isshielded by the narrow connecting region (in the aforementioned values,for example, 2 pm) between two shielding zones 403, so no potentialpunch-through occurs. This structure is therefore especially well-suitedfor high blocking voltages.

What is claimed is:
 1. A controllable semiconductor structure having abase region, a source region and a drain region, with a conductivechannel being provided in the base region between the source and thedrain, and wherein the channel can be pinched off by zones parallelthereto, including an active control zone and an oppositely-located,passive control zone, which respectively form a blockable transition tothe base region; a conductive connection exists between the passivecontrol zone and the source region; and the semiconductor material ofthe base region has a band gap of more than 1.2 eV.
 2. The semiconductorstructure according to claim 1, characterized in that the source region(106, 212, 312, 412) and the drain region (107, 213, 313, 413) aredisposed on oppositely-located surfaces.
 3. The semiconductor structureaccording to one of claims 1 through 3, wherein the active control zoneand the passive control zone are at potentials that are selectedindependently of one another.
 4. The semiconductor structure accordingto one of claims 1 through 3, wherein the expansion of the space-chargezone in the base can be controlled by the application of a voltagebetween the source and the electrode of the active control zone.
 5. Thesemiconductor structure according to claim 1 or 2, wherein anothersemiconductor zone extends in front of the drain region, with theanother zone having the opposite conductivity type of the base region.6. The semiconductor structure according to claim 5, wherein a furtherzone is disposed between the base region and the another zone, thefurther zone having the same conductivity type as the base region, but aheavier doping.
 7. The semiconductor structure according to one ofclaims 1 through 3, wherein the control-voltage difference that existsbetween the pinch-off of the conductive channel and the setting of thecontrol current increases with increase band gap of the semiconductormaterial of the base region.
 8. The semiconductor structure according toclaim 1, wherein the source and drain regions and the active controlzone are disposed on the same surface of the semiconductor component. 9.The semiconductor structure according to claim 1, wherein one of galliumarsenide, the different polytypes of silicon carbide, gallium nitride,diamond and aluminum nitride are provided as the semiconductor material.10. The semiconductor structure according to claim 9, wherein the activeand passive control zones, independently of one another, comprise thesame semiconductor material as the base region, a semiconductor materialthat differs from the base region material, or a metal.